checkkconfigsymblos: consistent symbol terminology

'symbol' and 'feature' are used synonymously to refer to Kconfig symbols
(configs, menus, etc.).  Use the term 'symbol' to have a consistent
terminology and to make the code more comprehensible.

Signed-off-by: Valentin Rothberg <valentinrothberg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>